The export is failing because an object referenced in another object's properties is missing or invalid.

Resolution:

Run XPSSweeper (without options/arguments) on the Policy Server to get more detail about the problem and a full list of problematic objects. XPSExport will typically stop upon the first fatal error, so using XPSSweeper gives a more complete view of the problem while also having the ability to auto-correct some problems.

In this case, XPSSweeper confirmed that this IMS object and one other IMS object contained links to objects that did not exist. We used XPSExplorer to locate the objects with bad links and delete those links. After these two updates, XPSSweeper reported no errors and the XPSExport succeeded.